Return a list of threads from HandlerList#dispatch

I have a plugin which wants to block on the execution of an event (being used as a hook to trigger other plugins to act). This pull request changes the return values of:

Handler#call: now returns the Thread it added to @thread_group
HandlerList#dispatch: now returns an array of Threads the event started

These methods previously returned void.

This allows this proof-of-concept plugin to run as expected: https://gist.github.com/nickrw/5769705